Commercially-available vehicles comprise a rotating electrical machine, or an alternator, with an internal ventilation or cooling system for passing air through the alternator. FIG. 1 illustrates one example of an existing alternator 101 comprising a rotor 102 and a shaft 104 extending through the center of the rotor 102, which is annular in shape. A front axial end of the shaft 104 is coupled to a drive pulley 106 that transfers movement from a vehicle engine (not shown) to the alternator 101. The alternator 101 further includes an internal cooling system comprising a front fan 112 axially coupled to the shaft 104 at a front end of the rotor 102, and a rear fan 114 axially coupled to the shaft 104 at a rear end of the rotor 102.
Because the shaft 104 is directly coupled to both fans 112 and 114 and to the rotor 102, rotation of the shaft 104 causes a corresponding rotation of the rotor 102 and both of the fans 112 and 114. As a result, the fans 112 and 114 rotate at the same speed as the alternator shaft 104. Moreover, because the shaft 104 of the alternator 101 is driven by the vehicle engine (via the drive pulley 106), the fan speeds are also related to the engine speed. For example, when the engine is idle, the fans 112 and 114 operate at their lowest operation speed (e.g., 1625 rpm). And when the engine speed is high (e.g., 5000 rpm), the fans 112 and 114 are operating at high operation speeds (e.g., 12,500 rpm) as well (for example, as shown by curve B in FIG. 11).
When the alternator fans operate at high speeds, the airflow produced by the fans can become excessive and generate significant radiated noise, resulting in a Noise, Vibration, and Harshness (NVH) penalty. At the same time, alternators have been increasing in size to support increasing vehicular electrical loads, and as a by-product, the internal heat loads for alternators have also increased. Thus, today's alternators require larger fans to provide adequate cooling, but also need to operate at very high speeds to accommodate the higher electrical loads.